Controlling carbon nanotubes using optical traps

ABSTRACT

An embodiment of the present invention is a technique to control carbon nanotubes (CNTs). A laser beam is focused to a carbon nanotube (CNT) in a fluid. The CNT is responsive to a trapping frequency. The CNT is manipulated by controlling the focused laser beam.

BACKGROUND

1. Field of the Invention

Embodiments of the invention relate to the field of semiconductor, andmore specifically, to nanotechnology.

2. Description of Related Art

Carbon nanotubes are promising elements in nanotechnology. They arefullerene-related structures which consist of graphene cylinders. Carbonnanotubes can be functionalized (by attaching moieties to nanotubes) toincrease their solubility in solvents and to control their affinity withother molecules or solid materials.

Current methods for nanotube separation are centrifuge and liquidchromatography based on chemical affinity. Trapping is done on the edgeof electrode in dielectrophoresis. For nanotube manipulation, currentmethods are based on scanning probe microscope and direct current (DC)or alternate current (AC) electrical field alignment. These techniquesare not precise and flexible to be used in a variety of applications.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention may best be understood by referring to thefollowing description and accompanying drawings that are used toillustrate embodiments of the invention. In the drawings:

FIG. 1 is a diagram illustrating a system in which one embodiment of theinvention can be practiced.

FIG. 2 is a diagram illustrating manipulation of CNTs according to oneembodiment of the invention.

FIG. 3 is a diagram illustrating manipulation of CNTs using layers withdifferent viscosities according to one embodiment of the invention canbe practiced.

FIG. 4 is a diagram illustrating manipulation of CNTs using polarizedlaser beam according to one embodiment of the invention.

DESCRIPTION

An embodiment of the present invention is a technique to control carbonnanotubes (CNTs). A laser beam is focused to a carbon nanotube (CNT) ina fluid. The CNT is responsive to a trapping frequency. The CNT ismanipulated by controlling the focused laser beam.

In the following description, numerous specific details are set forth.However, it is understood that embodiments of the invention may bepracticed without these specific details. In other instances, well-knowncircuits, structures, and techniques have not been shown in order not toobscure the understanding of this description.

One embodiment of the invention may be described as a process which isusually depicted as a flowchart, a flow diagram, a structure diagram, ora block diagram. Although a flowchart may describe the operations as asequential process, many of the operations can be performed in parallelor concurrently. In addition, the order of the operations may bere-arranged. A process is terminated when its operations are completed.A process may correspond to a method, a procedure, a method ofmanufacturing or fabrication, etc.

FIG. 1 is a diagram illustrating a system 100 in which one embodiment ofthe invention can be practiced. The system 100 includes a laser 110, anoptical assembly 120, and a fluid 130.

The laser 110 focuses a laser beam 125 to the chamber 130 through theoptical assembly 120. The laser 110 may be controlled to have a numberof operating modes. It can be controlled to have varying intensities andoptical frequencies. It may be polarized.

The optical assembly 120 provides optical elements to process the laserbeam 125. Examples of the optical elements includes diffractive optics,lenses, telescopic lenses, optical modulators, and filters. The opticalassembly 120 directs the laser beam 125 to carbon nanotubes (CNTs) inthe fluid 130.

The fluid 130 includes multiple layers or channels of different fluids.It may be contained in a fluidic channel or container made by glass or apolymeric material. The fluid 130 includes a number of CNTs 135 ₁ to 135_(N). The CNTs may be single-walled CNTs (SWNTs) or multi-walled CNTs(MWNTs). The CNTs may be functionalized.

The system 100 allows controlling the CNTs in the fluid 130 in a numberof ways. The manipulation includes trapping a certain class of CNTs,moving a trapped CNT, releasing a trapped CNT, and aligning the trappedCNTs. The use of the laser 110 to manipulate the CNTs is based on theconcept of optical dipole traps.

A focused laser beam can trap a neutral particle or molecule through theinteraction between the electric field of the laser beam and thespontaneous dipole momentum induced in the particle or molecule. Theinduces dipole momentum of a neutral particle in the electric filed of alaser beam can be expressed as:P=ε_(0ψ)E  (1)

where P is the polarization or dipole momentum per unit volume, ε₀ isthe permittivity of free space, ψ is the dielectric susceptibility, andE is the electric field.

The potential energy may be expresses as:U=(−½)<P·E>=(−½)ε_(0ψ) <E> ²  (2)

The dielectric susceptibility may be expressed in a complex form as afunction of frequency as follows:ψ(ω)=ψ′(ω)+iψ″(ω)  (3)

where ψ′(ω) is the real part and ψ″(ω) is the imaginary part.

When ω<ω₀, ψ′(ω)>0, where ω₀ is a resonant frequency.

From equation (2), it is derived that the potential energy U decreaseswhen the light intensity increases. Furthermore, the particle tends tomove to an area of higher E and is trapped at the center of a laserbeam, assuming that the optical intensity distribution of the laser beamis Gaussian.

Depending on the diameter and chirality, a SWNT may be metallic orsemiconductor. The electron density of states of a SWNT is composed ofmany spikes, called the van Hove singularities. The energy gaps betweenthe corresponding van Hove singularities are optically allowedinter-band transition energies. By choosing a proper laser frequency orcontinuously tuning the laser frequency, a certain type of nanotubes canbe trapped. A MWNT is an assembly of multiple SWNTs with differentdiameters and chiralities. The trapping of a MWNT depends on itscomposition, i.e., ratio of different SWNT types. A laser frequency thatcan trap all types of SWNTs can also trap MWNTs.

The nanotubes can also be aligned using a polarized laser beam. Thedipole is always parallel to the axis of the nanotube. The polarizationP may be decomposed into a parallel component P_(p) and an orthogonalcomponent P_(o):P=P _(p) +P _(o) ≅P _(p)=ε_(0ψ) E _(p)  (4)

where E_(p) is the parallel component of E.

The potential energy can then be expressed as:U=(−½)<P·E>=(−½)<E _(p)>² cosθ  (5)

where θ is the angle between E and the axis of a CNT.

From the above equations, an increase in E leads to a decrease in U.Also, a decrease in θ leads to a decrease in U when ω<ω₀, ψ′(ω)>0.Therefore, CNTs can be trapped and aligned by polarized laser beam

FIG. 2 is a diagram illustrating manipulation of CNTs according to oneembodiment of the invention. The fluid 130 includes a first layer 210, abuffer layer 220, and a second layer 230.

The three layers 210, 220, and 230 are laminar flow layers. The bufferlayer 220 prevents the random diffusion of the CNTs between the firstand second layers 210 and 230. The first layer 210 contains a number offree CNTs 135 ₁ to 135 _(N).

The laser beam is focused to capture the CNT 135 _(k) at the site 240 inthe first layer 210. The laser beam is focused with a specificfrequency, referred to as a trapping frequency, to selectively trapand/or release the CNT 135 _(k) that is responsive to this trappingfrequency. Once the CNT 135 _(k) is trapped, it can be moved andreleased by controlling the laser beam.

To move the CNT 135 _(k), the position of the focal point of the laseris changed from the site 240 to a site 250 in the second layer 230. Thelaser can be precisely moved and therefore the movement of the CNT 135_(k) can be precisely controlled. Once the CNT 135 _(k) is moved to anew location, it can be released.

The trapped CNT 135 _(k) can be released at either the first layer 210or the second layer 230 at any location, e.g., the sites 240 or 250using a number of methods. In the first method, the laser 110 is simplyturned off, cutting off the laser beam. The electric field is removedand the CNT 135 _(k) becomes free. In the second method, the laser beamis blocked, either by an optical or mechanical blocker. In the thirdmethod, the laser intensity is reduced by using a filter in the opticalassembly 120 or in the laser 110 itself. In the fourth method, thefrequency of the laser 110 is changed to be different than the trappingfrequency. In the fifth method, the fluid at the second layer 230 isreplaced with another fluid with different viscosity or dielectricconstant from the first layer 210. In the sixth method, the laser beammoves across a liquid-solid interface (e.g. the wall of a microfluidicchannel).

The trapping, moving, and release of the CNTs may be performedcontinuously by synchronizing the sweeping of the laser beam and theevent of releasing the CNTs.

FIG. 3 is a diagram illustrating manipulation of CNTs using layers withdifferent viscosities one embodiment of the invention can be practiced.The fluid has three layers 310, 320, and 330.

The laser beam is focused to the CNTs in the first layer 310. The CNT135 is optically trapped at a site 340. The trapped CNT 135 can be movedto the second layer 320 at a site 350 by moving the laser beam. Then,the trapped CNT 135 is released at the interface of the two laminar flowlayers 320 and 330 with different viscosities, when the shear force onthe CNT 135 due to the third layer 330 is larger than the laser trappingforce.

The laser beam may sweep back and forth between the site 340 and a site360 in the third layer 330 to trap, move, and release the CNT 135 at theinterface between the second and third layers 320 and 330. Thistechnique does not require a modulation of the laser intensity, orvarying of frequency to release CNTs.

An extreme case in FIG. 3 is that the layer 330 is a solid (e.g. a wallof a microfluidic channel).

FIG. 4 is a diagram illustrating manipulation of CNTs using polarizedlaser beam according to one embodiment of the invention. The fluid 130includes first, and second layers 410 and 420. A layer 440 is a solidsubstrate (e.g., glass, silicon). An adhesion layer 430 may be coated onthe substrate 440.

The CNT 135 is trapped by the laser beam at a site 450 at the firstlayer 410. The CNT 135 is responsive to a polarization. The laser is apolarized laser. The CNT 135 is aligned to the orientation as providedby the polarized laser beam. The trapped CNT 135 is moved to a site 460at the second layer 420 by changing the position of the laser focalpoint accordingly. The trapped CNT 135 is then released at the surfacebetween the second layer 420 and the adhesion layer 430. The layer 430provides support for the released CNT 135.

The layer 430 also immobilizes the CNT 135 while keeping its alignmentor orientation the same as the laser polarization direction. The layer430 can be patterned by lithography method to further define thelocation to where CNT 135 can attach.

If the trapped CNT 135 has high affinity with the surface of thesubstrate 440, the adhesion layer 430 is not necessary and theimmobilization of the CNT 135 may be performed by the substrate layer440.

The role of the second layer 420 is to prevent CNT from randomlydiffusing on to the layer 430 or layer 440. If the CNT concentration inthe first layer 410 is dilute enough that the non-specific binding onthe surface 430 or 440 is negligible, the second layer 420 is notnecessary.

The surface of the substrate layer 440 may be functionalized toimmobilize the CNT 135 while keeping its alignment or orientation thesame. This can be done by a number of ways. For example, the substratelayer 440 may be coated with a layer 430 of positively charged molecules(e.g. self-assembled 3-Aminopropyltriethoxysilane monolayer) that canbind to the CNT 135 or the functional group or chemical moiety onfunctionalized CNT 135 when it is near the surface.

The laser beam may sweep back and forth between the site 450 and a site470 in the substrate layer 440 to trap, align, move, release, anddeposit (immobilize) the CNT 135 on the substrate layer 440.

While the invention has been described in terms of several embodiments,those of ordinary skill in the art will recognize that the invention isnot limited to the embodiments described, but can be practiced withmodification and alteration within the spirit and scope of the appendedclaims. The description is thus to be regarded as illustrative insteadof limiting.

1-8. (canceled)
 9. An apparatus comprising: a fluid containing a carbon nanotube (CNT), the CNT being responsive to a trapping frequency, the CNT being manipulated by a focused laser beam controlled by a laser.
 10. The apparatus of claim 9 wherein the laser varies frequency of the laser beam to match the trapping frequency to trap the CNT at a trapping site in a first fluidic layer.
 11. The apparatus of claim 10 wherein the laser changes position of focal point of the laser beam from a first location to a second location to move the trapped CNT from a first site in the first layer to a second site in a second layer.
 12. The apparatus of claim 10 wherein the trapped CNT is released by one of turning off the laser beam, blocking the laser beam, filtering the laser beam to reduce laser intensity, changing frequency of the laser beam, using a different fluidic medium at the second layer having different viscosity or dielectric constant, and moving the laser beam across a liquid-solid interface.
 13. The apparatus of claim 10 wherein the trapped CNT is released at an interface between the first and second layers having different viscosities.
 14. The apparatus of claim 10 wherein the trapped CNT is aligned by polarizing the laser beam.
 15. The apparatus of claim 9 further comprising: a substrate layer to immobilize the CNT by being functionalized.
 16. The apparatus of claim 9 wherein the laser beam is focused to a multi-walled CNT (MWNT) having a plurality of single-walled CNT (SWNT) to manipulate the MWNT.
 17. A system comprising: a fluid containing a carbon nanotube (CNT), the CNT being responsive to a trapping frequency; a laser to focus a laser beam to the CNT to manipulate the CNT by controlling the laser beam; and an optical assembly placed between the laser and the fluid to modulate the laser beam.
 18. The system of claim 17 wherein the laser varies frequency of the laser beam to match the trapping frequency to trap the CNT at a trapping site in the first fluidic layer.
 19. The system of claim 18 wherein the laser changes position of focal point of the laser beam from a first location to a second location to move the trapped CNT from a first site in the first layer to a second site in the second layer.
 20. The system of claim 18 wherein the trapped CNT is released by one of turning off the laser beam, blocking the laser beam, filtering the laser beam to reduce laser intensity, changing frequency of the laser beam, using a different fluidic medium at the second layer having different viscosity or dielectric constant, and moving the laser beam across a liquid-solid interface.
 21. The system of claim 18 wherein the trapped CNT is released at an interface between the first and second layers having different viscosities.
 22. The system of claim 18 wherein the trapped CNT is aligned by polarizing the laser beam.
 23. The system of claim 17 further comprising: a substrate layer to immobilize the CNT by being functionalized.
 24. The system of claim 17 wherein the laser beam is focused to a multi-walled CNT (MWNT) having a plurality of single-walled CNTs (SWNTs) to manipulate the MWNT. 